The present disclosure relates to a battery system which has a battery which comprises a plurality of battery cells and which can be connected on the input side to a direct voltage intermediate circuit via at least one contactor, and a diagnostic device for diagnosing the state of the at least one contactor. In addition, the disclosure relates to an associated method for diagnosing the state of battery contactors.
It has become apparent that in future new battery systems of which very stringent demands are made in terms of reliability will be increasingly used both in stationary applications, such as, for example, in wind power plants, and in motor vehicles, such as, for example, in hybrid vehicles and electric vehicles. The background to these stringent demands is that failure of the battery of a battery system which is used can lead to a safety-related problem. The basic circuit diagram of such a battery system 10 is illustrated in FIG. 1. In order to achieve the required performance and energy data by means of the battery system 10, individual battery cells 21 are connected in series and partially additionally in parallel in the battery (battery pack) 20 of the battery system 10. In order to simplify the illustration, only a single battery cell with a reference symbol 21 has been provided in FIG. 1. A problem of the approach shown in FIG. 1, in which a large number of battery cells 21 are connected in series, is the high output voltage UB of a battery (high voltage battery) 20 which occurs with this series connection, which output voltage UB is continuously present at the on-board power system (high-voltage on-board power system) 40 of a motor vehicle unless suitable measures are used. The voltage, referred to as the “link voltage”, which is present at the on-board power system 40 has been denoted by UBN in FIG. 1. In the battery system 10 illustrated in FIG. 1, the battery 20 is connected to the on-board power system 40 of the motor vehicle by means of the capacitor 30 of a direct voltage intermediate circuit.
Because of the high voltage UBN, which is continuously present at the on-board power system 40 of the motor vehicle unless suitable measures are used, as a rule contactors 50, 60 are used which, when necessary, disconnect the battery 20 from the on-board power system 40. For safety reasons, a separate contactor 50, 60 should be respectively present both at the positive pole of the battery 20 and at the negative pole, which contactors 50, 60 are designed for a high battery voltage UB and are, under certain circumstances, also capable of disconnecting a short circuit current of over 1000 A. The contactor 50 is connected at its pole 51 to the battery 20, and at its pole 52 to the on-board power system 40. The contactor 60 is connected at its pole 61 to the battery 20, and at its pole 62 to the on-board power system 40.
So that the disconnection of the battery 20 with its high battery voltage UB from the on-board power system 40 is ensured, one of the previously described safety requirements involves checking the function of the contactors 50, 60 and reliably diagnosing a malfunction. Therefore, a particularly hazardous malfunction of the contactors 50, 60 which has to be diagnosed is, for example, if the contactors 50, 60 or the contactor contacts thereof stick during correct actuation and have not opened as actuated. The contactors 50, 60 may also have been destroyed to such an extent that they have not closed as actuated. For the implementation of diagnostic functions it is necessary to use suitable devices and algorithms.
According to the present prior art, topologies are known for implementing anti-sticking diagnostic functions which are embodied as a circuit device for implementing voltage measurements at the four contact poles of the two contactors and which are respectively arranged at the positive or the negative battery pole, between the corresponding battery pole and the on-board power system. Such circuit topologies are known, for example, from document EP 2 308 714 A2. The known circuit topologies may also include reference voltages which are connected when there are certain diagnostic configurations. In the case of open contactors, the potential differences between the four poles of the two contactors are evaluated. These potential differences are predefined, to a certain extent, by means of defined impedances or resistance dividers/voltage dividers.
The methods which are known from the prior art for diagnosing a sticking state of contactors which use considerations of voltage differences as a function of the switched states have the disadvantage that they are very sensitive to voltage cross-couplings between the pole potentials. In addition, these methods are very sensitive to changes in impedance between the battery poles and the on-board power system poles. Furthermore, a complete diagnosis requires a switching frequency to be implemented with simultaneous evaluation of voltage differences. Such a sequence is run through as a rule either at the start or at the end of a driving cycle. Since switching times have to be complied with for this purpose and transient recovery processes play a role, starting delays often occur, depending on the method, specifically until the diagnosis is completed.
Furthermore, document DE 10 2004 041 998 A1 discloses a method for predicting the operational capability of a relay or of a contactor in which a current which flows through the relay or the contactor is measured repeatedly. In the same method, a current temperature of the relay of the contactor is estimated by means of the measured current values and on the basis of known current temperature characteristic curves of the relay or of the contactor. A prediction about the operational capability, in particular about a sticking state, of the relay or of the contactor is then made by means of the estimated temperature.